The majority of steering rack bars are manufactured from a cylindrical bar of steel having cut therein teeth over about one quarter of the length extending from one end. The shortcomings of racks produced by this technique are described in U.S. Pat. Nos. 4,715,210 and 4,571,982 which describe a method and apparatus respectively for making steering rack bars by forging in a multi-element die commonly known as a "Y-Die" in which the forming elements of the die converge towards the centre line of the rack bar in order to maximise the forming pressure and produce minimal "flash". It is particularly suited to producing racks having a unique cross section through the toothed portion of the rack bar which resembles the capital letter "Y", which has significant advantages as described in U.S. Pat. No. 4,116,085.
Rack bars produced by the apparatus described in U.S. Pat. No. 4,571,982 have superior bending and fatigue strength to rack bars made from the same diameter cylindrical bar stock and the forging process permits either constant or variable ratio tooth forms, as described in U.S. Pat. No. 3,753,378, to be imparted. Variable ratio tooth forms, wherein the ratio curve changes smoothly over the axial extent of the rack travel, can not be accurately produced by broaching or grinding and can not be economically produced by methods other than forging, eg: chemical or electro discharge machining.
Rack bars have been produced by the "warm" forging technique described in U.S. Pat. No. 4,571,982 since 1983. A feature of that die is that the die cavity volume is closely matched to the volume of the blank so that minimal "flash" is produced. Although the claimed benefits of superior fatigue and bending strengths about principal axes, superior straightness of product, lower cost production and ability to produce variable ratio tooth forms have all been realised, production experience has highlighted a number of shortcomings in the design of the current form of Y-die.
Principal amongst the shortcomings of the current Y-die is the inability to independently control the forging and gripping loads. In this prior art die, the upper die element (controlling forging load) and upper gripper (controlling gripping load) are each attached to a single plate and pre-loaded vertically downwards by two springs. This plate is vertically slideable in the upper platen and hence independent motion of the upper die element and upper gripper is impossible. The upper die element serves to volumetrically contain the formed metal rising in the stem of the Y-form cross section of the rack and has been found in practice to rise to a varying (but slight) degree in order to accommodate the diametral tolerance of the blank. This results in the plate, to which the upper die element and upper gripper are attached, also rising to a varying degree and therefore producing variability in the gripping loads.
Moreover the uneven load distribution on the plate causes the side of the plate adjacent to the toothed portion of the rack to deflect vertically upward relative to the side supporting the upper gripper, tending to prise the upper and lower grippers open. This leads to a loss of gripping force and permits metal to be extruded axially between the grippers with attendant local loss of die pressure and consequent poor tooth fill. The upward relative deflection of the plate further unbalances the axial pressure distribution in the die cavity, often necessitating a number of iterations on the dimensions of the upper die element until satisfactory tooth fill and Y-form cross-section have been achieved. This process, which must be carried out after each change of tooling, can be time consuming and is consequently not suitable for a high volume production environment where rapid changeover of tooling is required.
A further consequence of the uneven deflection of the abovementioned plate is a lack of straightness in forged rack bars, which manifests as a bend at the transition between the toothed and cylindrical portions of rack bars.
A further shortcoming of the current design of Y-die is in the use of mechanical springs to control forging and gripping forces. The helical coil springs, as illustrated in U.S. Pat. No. 4,571,982, are difficult to package and for this reason mechanical beam springs have been used to date in production of Y-dies. However the abovementioned spring pre-load is not readily varied in such mechanical springs, whether of coil or beam type, and varies in service due to wear, thereby introducing variability to the process and, by loss of mechanical pre-load, increasing the likelihood of a fatigue failure in the springs. Also, different sizes of rack bars may require different maximum spring loads and/or spring rates and these parameters are again not readily varied in a production environment.
The present invention provides a die suited to the forming of steel rack bars of the configuration described in U.S. Pat. No. 4,571,982 without the shortcomings of the prior art of Y-die. An important feature of the present invention is the provision of separate control of gripping and forging forces and pressures. This separation of gripping and forging functions permits optimisation of each with attendant improvement of product tooth fill and rack bar straightness. Furthermore the return of the upper die element and the upper and lower grippers from the positions occupied at the moment of full die closure may be independently controlled and timed so as to release the forged rack from contact with the other die elements in a manner which avoids significant distortion and misalignment. Also the invention makes possible rapid fine tuning or re-establishment of forging parameters without having to dismantle the die, thereby facilitating rapid changeover of tooling and making the die suitable for use in a high volume production environment.